SPMetal is a command line tool that generates entity classes that are in turn used in Linq to SharePoint - Microsoft.SharePoint.Linq ( new in SP 2010 ). It is like an interface to connect to the Foundation 2010 Content Databases that enables operations such as add, delete and change items.
SPMetal can be found under the location C:\Program Files\Common Files\Microsoft Shared\web server extensions\14\BIN\
SPMetal /web:http://ContosoServer/Marketing /code:MarketingSite.cs
Reference : MSDN